FIG. 7 is a perspective view showing a current detection apparatus 90 disclosed in JP-A-2002-296305 and used to measure current of an in-vehicle battery.
The current detection apparatus 90 includes a ring-shaped magnetic core 20 having a center opening and a gap 20g and a magnetic sensor 30 arranged in the gap 20g. 
As indicated by a dashed line in FIG. 7, a busbar 10 is inserted through the center opening of the magnetic core 20 and surrounded by the magnetic core 20. When current to be measured flows through the busbar 10, magnetic flux is generated in the magnetic core 20. Thus, the magnetic core 20 acts as a magnetic flux path.
As shown in FIG. 8A, the magnetic sensor 30 has a power terminal 30v, a ground terminal 30g, and an output terminal 30s. The magnetic sensor 30 covers a predetermined measurement range and outputs an electrical signal having a value corresponding to intensity of magnetic field in the gap 20g. The current detection apparatus 90 measures the current based on the electrical signal output from the magnetic sensor 30. As shown in FIGS. 8B and 8C, the magnetic sensor 30 is constructed as a Hall effect integrated circuit (IC) such that a Hall effect element 30h and a peripheral circuit are packaged in a single IC. For example, the peripheral circuit includes a temperature detection element (TMP) such as a resistor, an analog multiplexer (MUX), an amplifier (AMP), a A/D converter (ADC), a digital signal processor (DSP), a D/A converter (DAC), a read only memory (ROM), and an a voltage follower circuit (VFC) as an output circuit. Thus, an output of the Hall effect element 30h is amplified and corrected for temperature dependence by the peripheral circuit and then output from the magnetic sensor 30.
For example, as disclosed in JP-A-2004-31170, there is a proposal that such a current detection apparatus of FIG. 7 should be used for battery charge control that prevent the battery from being charged under acceleration to improve fuel efficiency, or used for battery condition monitor that monitors condition of a battery by calculating an internal resistance of the battery.
In the battery charge control, battery current is controlled in a range between plus (charge) and minus (discharge) tens of amperes (A). A center point of the range is a zero-point (0 A) where no battery current flows. If a point where the battery current flows is used as the zero-point for a long time, the battery may be overcharged or overdischarged. In the battery charge control, therefore, the battery current needs to be measured very accurately.
In the battery condition monitor, the internal resistance of the battery is calculated as a slope of volt-ampere characteristics of the battery. Therefore, as the internal resistance is calculated from current values ranging widely, accuracy of the calculated internal resistance may be improved. Because discharging current flowing when an engine starts reaches a few hundreds of amperes, an accurate value of the internal resistance can be obtained by calculating the internal resistance from the discharging current and a battery voltage corresponding to the discharging current.
As described below, however, one current detection apparatus 90 alone cannot achieve both the battery charge control and the battery condition monitor.
For example, the current detection apparatus 90 outputs an analog voltage ranging from 0 V to 5V. The analog voltage is converted to digital signal used for various control purposes. If the analog voltage ranging from 0 V to 5 V is converted into 10-bit digital signal, one least significant bit (1LSB) of the digital signal has a weight of approximately 4.9 mV.
If the current detection apparatus 90 is used only for the battery charge control, the current detection apparatus 90 needs a current measurement range from −100 A to +100 A, for example. The current measurement range from −100 A to +100 A is converted into a voltage output range from 0.5 V to 4.5 V as follows:
      VOLTAGE    CURRENT    =                              4.5          ⁢          V                -                  0.5          ⁢          V                                      100          ⁢          A                -                  (                                    -              100                        ⁢            A                    )                      =                            4          ⁢          V                          200          ⁢          A                    =              20        ⁢                                  ⁢                  mV          /          A                    
In this case, current of 1 A corresponds to voltage of 20 mV and the current detection apparatus 90 has a resolution of 0.25 A. Because the current detection apparatus 90 needs a resolution of approximately 1 A to perform the battery charge control, the battery charge control can be achieved by using the current detection apparatus 90 having the current measurement range from −100 A to +100 A.
In contrast, if the current detection apparatus 90 is used for both the battery charge control and the battery condition monitor, the current detection apparatus 90 needs a current measurement range of −1000 A to +100 A. The current measurement range from −1000 A to +100 A is converted into the voltage output range from 0.5 V to 4.5 V as follows:
      VOLTAGE    CURRENT    =                              4.5          ⁢          V                -                  0.5          ⁢          V                                      100          ⁢          A                -                  (                                    -              1000                        ⁢            A                    )                      =                            4          ⁢          V                          1100          ⁢          A                    =              3.6        ⁢                                  ⁢                  mV          /          A                    
In this case, the current of 1 A corresponds to voltage of 3.6 mV and the current detection apparatus 90 has a resolution of approximately 1.39 A. Therefore, as a result of reduction in resolution, the battery charge control cannot be achieved by using the current detection apparatus 90 having the current measurement range from −1000 A to +100 A.
As described above, one current detection apparatus 90 alone cannot meet opposite requirements of the battery charge control and the battery condition monitor. The opposite requirements can be met by using two current detection apparatus 90, i.e., one current detection apparatus 90 having a very accurate measurement range suitable for the battery charge control and the other current detection apparatus 90 having a wide measurement range suitable for the battery condition monitor.
However, use of the two current detection apparatus 90 doubles manufacturing cost and process and increases overall size and installation space.